Liquid crystal displays (LCDs) have become a common way to display information in electronic devices and computers. LCDs have advantages such as being thin and light (when compared to cathode ray tube displays) as well as being energy efficient. LCDs typically operate by regulating the transmission of light, wherein in one state, the transmission of light through a picture element is permitted while in a second state the transmission of light through the picture element is blocked.
An LCD is made up of a plurality of pixels (or segments) that can be turned on or off by applying a voltage potential across a common (or backplane) electrode and a select electrode that is associated with each pixel. The state of a pixel is determined by a root mean square voltage (Vrms) across its common electrode and select electrode. The voltage potential across the electrodes can energize a liquid crystal fluid so that it can either pass or block the flow of light. For example, when the Vrms is greater than a threshold voltage for the LCD, the pixel is ON. The pixel is OFF when the Vrms is less than the threshold voltage for the LCD. Furthermore, in order to prevent damage to the LCD since a DC voltage can deteriorate the liquid crystal fluid so that it can no longer be energized, there is a requirement that no DC offset be present across any and all pixels.
Since the state of each pixel can be independently controlled, each pixel can be driven by a signal that is provided by an interconnection. However, since many LCDs can have a very large number of pixels, sharing (multiplexing) a single connection between multiple pixels can be used to reduce the overall number of interconnections between an LCD and driver circuitry. For example, in an LCD with ⅓ multiplexing (a multiplex factor of 3), a single common electrode interconnection can be used to control the state of three pixels. It is not unusual for an LCD with a large number of pixels to have 1/64 or 1/128 (or higher) multiplexing, wherein a single common electrode interconnection can be used to control the state of 64 or 128 pixels.
A commonly used prior art technique to drive a signal that can be used to control the state the pixels of an LCD involves the use of analog output drivers and voltage charge pump circuitry to provide necessary multi-voltage level drive signals. The use of multi-voltage level drive signals can simplify the generation of drive signals for multiplexed LCDs as well as maximize a delta between Vrms ON and Vrms OFF in order to maximize LCD viewing contrast.
Another prior art technique that can be used to control the state of the pixels of an LCD is to drive these pixels directly with logic-level circuitry. The use of logic level signaling (typically a two level signal) permits the direct coupling of the LCD with the circuitry used to generate the drive signals.
One disadvantage of the prior art is that the use of analog output drivers and voltage charge pumps are typically more difficult and complex to integrate into an integrated circuit. The increased difficulty and complexity increases the cost of producing LCD control circuitry as well as potentially decreasing the reliability of the circuitry. Therefore, the use of analog output drivers and voltage charge pumps can result in a more expensive LCD drive system that is potentially less reliable.
A second disadvantage of the prior art is that the use of logic level signaling can result in a relatively small difference between on and off RMS voltage levels for controlling the state of a pixel, when compared to the difference achievable when using charge pumps and analog output drivers. With a small difference between the on and the off voltages, the contrast between a pixel in the on state and a pixel in the off state is small. Therefore, the visual quality of the LCD is not as good as when there is a large difference between the on and the off voltages.